Error D.73

Safety precautions:

- Do not open the boiler casing or attempt internal electrical/gas repairs unless you are a qualified Gas Safe engineer.

- If you need to isolate the boiler for safety, switch it to standby using the power button and turn off the gas isolator cock if you are competent to do so. If in doubt, call a professional.

Initial checks a homeowner can do:

- Note exactly what the display shows (D.73 and any other symbols or fault codes). Make a note of times and circumstances when it appears (e.g. after power cut, after changing settings).

- Check the basic settings via the boiler menu: ensure the heating target flow temperature is set to a normal value (Vaillant recommends around 60–65 °C for central heating flow temperature). If it is set unusually low or high, return it to a reasonable value and confirm the setting.

- Check Comfort/preheat mode (if fitted) and the hot water temperature settings. Toggle Comfort mode off and on to see if that clears the condition.

- Check system pressure on the display. If the display also shows F22 (low pressure), top up the heating system using the filling loop following the boiler instructions. Low pressure can cause other faults; bring pressure to the normal cold level (typically around 1–1.5 bar) before further testing.

Simple reset and menu actions to try:

1. Try a simple reset: press and hold the power button for about 5 seconds to reset the boiler and see if D.73 clears. Watch for other fault messages after reset.

2. If D.73 remains, go into the boiler menu and check the heating flow target temperature. Use the panel controls to set the target flow temp to a normal value (e.g. 60–65 °C) and press OK to confirm. Note the previous value before changing it.

3. Toggle Comfort/preheat: if your boiler has Comfort mode, switch it off, confirm, then switch it on again and confirm. Observe if the D.73 message clears after cycling the mode and a reset.

4. Cycle power: turn the boiler to standby, wait 30 seconds, then power it back on and observe behaviour.

When to call a professional and what they will check:

- If D.73 persists after resets and correcting obvious menu values, call a Gas Safe registered heating engineer. Tell them the boiler model (Vaillant ecoTEC Plus), the code D.73, and what you have already tried (reset, changed target temperature, checked pressure, Comfort mode toggled).

- A competent engineer will check that the stored warm-start parameter is correct, verify sensor readings and wiring (flow sensor, pressure sensor and any temperature sensors involved), inspect for corroded/loose connectors, and if necessary check the PCB for incorrect stored values or faults. They may also check related settings/diagnostic parameters in the service menu that are not accessible to homeowners.

Additional notes:

- Do not attempt to change service parameters or access the inside of the boiler unless you are trained and Gas Safe registered. Incorrect adjustments can cause improper operation or create safety risks.

- Keep records of when the fault appears and any other fault codes shown; this helps the engineer diagnose faster.

- While D.73 is not normally an immediate safety shutdown, if other fault codes (ignition faults, pressure faults) appear simultaneously, treat those as higher priority and contact an engineer promptly.